The bacterial Sm-like protein Hfq forms homohexamers both in solution and in crystals. The monomers are organized as a continuous -sheet passing through the whole hexamer ring with a common hydrophobic core. Analysis of the Pseudomonas aeruginosa Hfq (PaeHfq) hexamer structure suggested that solvent-inaccessible intermonomer hydrogen bonds created by conserved amino-acid residues should also stabilize the quaternary structure of the protein. In this work, one such conserved residue, His57, in PaeHfq was replaced by alanine, threonine or asparagine. The crystal structures of His57Thr and His57Ala Hfq were determined and the stabilities of all of the mutant forms and of the wild-type protein were measured. The results obtained demonstrate the great importance of solvent-inaccessible conserved hydrogen bonds between the Hfq monomers in stabilization of the hexamer structure.

Small regulatory RNA (sRNA) is a unique noncoding RNA involved in regulation of gene expression in both eukaryotic and bacterial cells. This short review discusses examples of positive and negative...

The Sm and Sm-like proteins are widely distributed among bacteria, archaea and eukarya. They participate in many processes related to RNA-processing and regulation of gene expression. While the function of the bacterial Lsm protein Hfq and eukaryotic Sm/Lsm proteins is rather well studied, the role of Lsm proteins in Archaea is investigated poorly. In this work, the RNA-binding ability of an archaeal Hfq-like protein from Methanococcus jannaschii has been studied by X-ray crystallography, anisotropy fluorescence and surface plasmon resonance. It has been found that MjaHfq preserves the proximal RNA-binding site that usually recognizes uridine-rich sequences. Distal adenine-binding and lateral RNA-binding sites show considerable structural changes as compared to bacterial Hfq. MjaHfq did not bind mononucleotides at these sites and would not recognize single-stranded RNA as its bacterial homologues. Nevertheless, MjaHfq possesses affinity to poly(A) RNA that seems to bind at the unstructured positive-charged N-terminal tail of the protein.